Epoxy-resin systems resistant to aging, molded materials and components generated from them, and their utilization.
To protect them from environmental influences, electrical and electronic components are potted or encapsulated with reaction resins through the application of casting-resin methods. The intention is to provide protection against mechanical, chemical and climatic environmental influences, particularly protection against impact, against breakage, against aggressive chemicals and against moisture, where such protection must be upheld at high environmental temperatures, which can reach approximately 130 degrees Celsius in the engine area of an automobile. Optoelectronic components, especially light-emitting components such as light-emitting diodes (LEDs), are exposed to particular stress, because the resin system used for encapsulation is subjected to additional radiation stress coming from the LED itself. In addition to having chemical and mechanical resistance, as well as a high degree of compound strength with a variety of substrates, such reaction-resin molded material must have a high degree of transparency, with an index of refraction nD greater than 1.5, if possible, and it must also exhibit resistance to light in terms of clouding, embrittlement and yellowing. The problem of increased radiation stress in light-emitting semiconductor components is amplified especially at decreasing wavelengths, increasing radiation doses of the component and the amplification of the radiation intensity in so-called high-power LEDs emitting at longer wavelengths, and for blue-emitting semiconductor components, for example, this results in the continuing lack of success in finding a satisfactory resin system resistant to aging that can be used for their encapsulation.
Commercially available LEDs are currently potted with two-component thermally cross-linking reaction resins based on epoxides, with anhydrides as hardeners. These substances are Theologically optimized and exhibit thermoreactive properties that allow for rational mass production. There the epoxy-resin component (A component) of these two-component reaction-resin systems consists of a low to medium-viscosity bisphenol A diglycidyl ether and a reactive epoxy diluent or polyfunctional epoxy novolak resins, as well as the additives and auxiliary substances common in casting resins. When used in LEDs, the A component may also be colored with organic pigments. Diffusely light-emitting components can exhibit special diffusor pastes based on inorganic pigments. With the aid of specific metal-salt accelerators containing organic carboxylate and alcoholate anions or ligands, two-component reaction-resin systems on anhydride/epoxide basis can be hardened. For the encapsulation of optoelectronic components, there are also two-component anhydride/epoxy casting-resin compositions based on cycloaliphatic epoxy resins and their blends with glycidyl-ether and glycidyl-ester resins. In special cases, acrylic-based encapsulants are also used.
However, it can be seen that the known casting compositions are not constructed to be sufficiently age-resistant to conform to the requirements for encapsulation of high-power LEDs, in particular, which are becoming ever more demanding. Even within the useful lives of the components, they already begin to exhibit clouding and yellowing, and can also exhibit brittleness, resulting in increasing porosity, as well as cracks, tears and reduced compound strength with the light-emitting component, including delamination. But molded materials from casting resins containing damage of this type result in a reduced light yield resulting in reduced optical-radiation characteristics, rendering them unsuitable for components in use constantly or according to customer specifications.